Extraction of tungsten by amines has been known for a number of years as reported by Coleman, D.F., et al., "Amines as Extractants", Nuclear Science and Engineering, Vol. 17, 274-286, 1963. C.R. Kurtak in U.S. Patent No. 3, 158,438, 1964, describes an integrated solvent extraction process using an amine. A long chain aliphatic alcohol is used as a phase conditioner (or solubilizer). P. Coad in U.S. Patent No. 3,804,941 describes tungsten recovery from an aqueous alkaline brine solution using a quaternary amine. Tungsten recovery from low grade concentrates containing low concentrations of tungsten using a primary amine and a quaternary amine is reported in Investigation 6845, U.S. Department of the Interior. A tricaprylyl amine, tributyl phosphate and kerosene system has also been used. A preferred kerosene is a material sold under the trade name of Kermac 470B by Kerr-McGee. In the above system tributyl phosphate is essential for the amine-tungsten complex to remain soluble in the organic extractant solution. Without tributyl phosphate the complex is insoluble and the systen cannot be operated. Generally a solution containing from about 10 to about 14 percent by volume of the tricaprylyl amine, from about 10 to about 14 percent by volume of tributyl phosphate and the balance kerosene is used. The tungsten is stripped from the organic by aqueous ammonia to form ammonium tungstate.
While the tricaprylyl amine-tributyl phosphate-kerosene system provides satisfactory recovery of tungsten it does have some disadvantages. In addition to the necessity of using tributyl phosphate to provide solubility for the amine-tungsten complex, sodium is extracted along with the tungsten. In addition, tributyl phosphate hydrolyzes to produce di-n-butyl phosphoric acid and mono-n-butyl phosphoric acid. These acids extract cationic impurities. It is also necessary in the above extraction system to add heat.
U.S. Pat. No. 4,175,109 describes a 2 component solvent extraction system consisting essentially of a tertiary amine dissolved in an aromatic solvent. This system is an improvement over the three phase system described above in that phase separation (disengagement) of organic and aqueous phases is greatly improved. Furthermore, there is increased throughput with this system and a product which is relatively sodium free.
In the above described systems, the tungsten is stripped from the organic by aqueous ammonia in an organic continuous dispersion to form ammonium tungstate. This ammonium tungstate is converted through evaporative heating to ammonium paratungstate (APT). For many applications, ammonium metatungstate, (AMT), is desired because it is a highly water soluble form of tungsten. In order to produce AMT, the APT is then roasted to drive off ammonia and/or water and is then digested to convert the APT to AMT. The resulting AMT solution is concentrated and solid AMT is crystallized from the solution. The steps of converting the ammonium tungstate via APT to AMT are labor intensive and therefore costly.
Therefore an improvement to the process which would result in direct conversion to AMT without first converting normal ammonium tungstate to APT would be highly desirable.